The total number of axons in each optic nerve was estimated using the nerve cross-sectional area and axon density as found in
Figure 5 . For this calculation, the mean density calculated from the high-magnification random sampling was assumed to represent a uniform density for each group, which is not necessarily the case. We found little variability in nerve area, which spanned a narrow range of 0.07 to 0.11 mm
2 for individual nerves, with no systematic difference between age groups (
P = 0.08). Accordingly, the distribution of total axon number by age in
Figure 6Afollows the trend for axon density in
Figure 5A : There are fewer total axons in the old high-IOP nerves than in the old low-IOP nerves (
P = 0.04), whereas the young high-IOP nerves had fewer axons than the old low-IOP nerves (
P = 0.02). Plotting total axon number against mean lifetime IOP
(Fig. 6B)shows that mean lifetime IOP is also a good predictor of total axon number (
r 2 = −0.53,
P = 0.003). As with axon density, the young high-IOP nerves have a number of axons similar to that of the old high-IOP nerves (
P = 0.52), owing to their similar IOP. Thus, for both axon density
(Fig. 5B)and number
(Fig. 6B) , mean IOP is a strong predictive factor. This is reflected in the positive correlation between axon total and axon density in
Figure 6C(
r 2 = 0.60, P ≪ 0.001).
Figure 6Cincludes additional data from the young normal group
(Table 1) . The mean axon density for these four eyes was 5.5 ± 0.2 × 10
5 axons/mm
2, which is statistically different from the mean for the two eyes from the young high-IOP group (
P = 0.005). Similarly, the total number of axons in the nerves from the young normal group is approximately 25% more (4.06 vs. 3.1 × 10
4 axons;
P = 0.008) than the number in the young high-IOP nerves.